The more popular pipettors in use presently are of two types: (1) solid displacement pipettors, and (2) non-solid displacement pipettors.
A solid displacement pipettor, as shown, for example, in U.S. Pat. No. 3,506,164, includes a piston which comes into direct contact with the liquid being sampled. This provides greater accuracy in drawing in and dispensing measured quantities of samples.
However, such pipettors must be carefully cleaned after each use to prevent contamination of subsequent samples to be analyzed. A further disadvantage of solid displacement pipettors is potential contamination by laboratory personnel who must handle the pipettors and/or clean them.
A non-solid displacement pipettor, as shown, for example, in U.S. Pat. No. 3,766,784, employs a disposable tip which provides an air space between the piston and the sample itself, such that the piston does not contact the sample. Such pipettors are relatively contamination-free, since laboratory personnel do not come into contact with the sample being analyzed during sampling or cleaning. In fact, cleaning is virtually unnecessary since no portion of the pipettor itself (as distinguished from the disposable tip) comes into contact with the sample.
However, pipetting with non-solid displacement pipettors is less accurate than pipetting with solid displacement pipettors. The compressibility of the air between the piston and the sample gives rise to the danger of inaccurate volume measurement, particularly where very small volumes are pipetted.
U.S. Pat. No. 4,084,730 and 4,249,419 show pipettors having disposable pistons. However, these pipettors are cumbersome and do not permit ready, contamination-free disposition and replacement of the tips and pistons.